Transistors can be divided into two main types: bipolar junction transistors and field-effect transistors. Both types share a common structure comprising three electrodes with a semiconductive material disposed therebetween in a channel region. The three electrodes of a bipolar junction transistor are known as the emitter, collector and base, whereas in a field-effect transistor the three electrodes are known as the source, drain and gate. Bipolar junction transistors may be described as current-operated devices as the current between the emitter and collector is controlled by the current flowing between the base and emitter. In contrast, field-effect transistors may be described as voltage-operated devices as the current flowing between source and drain is controlled by the voltage between the gate and the source.
Transistors can also be classified as p-type and n-type according to whether they comprise semiconductive material which conducts positive charge carriers (holes) or negative charge carriers (electrons) respectively. The semiconductive material may be selected according to its ability to accept, conduct, and donate charge. The ability of the semiconductive material to accept, conduct, and donate holes or electrons can be enhanced by doping the material. The material used for the source and drain electrodes can also be selected according to its ability to accept and inject holes or electrons. For example, a p-type transistor device can be formed by selecting a semiconductive material which is efficient at accepting, conducting, and donating holes, and selecting a material for the source and drain electrodes which is efficient at injecting and accepting holes from the semiconductive material. Good energy-level matching of the Fermi-level in the electrodes with the HOMO (Highest Occupied Molecular Orbital) level of the semiconductive material can enhance hole injection and acceptance. In contrast, an n-type transistor device can be formed by selecting a semiconductive material which is efficient at accepting, conducting, and donating electrons, and selecting a material for the source and drain electrodes which is efficient at injecting electrons into, and accepting electrons from, the semiconductive material. Good energy-level matching of the Fermi-level in the electrodes with the LUMO (Lowest Unoccupied Molecular Orbital) level of the semiconductive material can enhance electron injection and acceptance.
Transistors can be formed by depositing the components in thin films to form thin film transistors. When an organic material is used as the semiconductive material in such a device, it is known as an organic thin film transistor.
Various arrangements for organic thin film transistors are known. One such device is an insulated gate field-effect transistor which comprises source and drain electrodes with a semiconductive material disposed therebetween in a channel region, a gate electrode disposed adjacent the semiconductive material and a layer of insulating material disposed between the gate electrode and the semiconductive material in the channel region.
An example of such an organic thin film transistor is shown in FIG. 1. The illustrated structure may be deposited on a substrate (not shown) and comprises source and drain electrodes 2, 4 which are spaced apart with a channel region 6 located therebetween. An organic semiconductor 8 is deposited in the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4. An insulating layer 10 of dielectric material is deposited over the organic semi-conductor 8 and may extend over at least a portion of the source and drain electrodes 2, 4. Finally, a gate electrode 12 is deposited over the insulating layer 10. The gate electrode 12 is located over the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4.
The structure described above is known as a top-gate organic thin film transistor as the gate is located on a top side of the device. Alternatively, it is also known to provide the gate on a bottom side of the device to form a so-called bottom-gate organic thin film transistor.
An example of such a bottom-gate organic thin film transistor is shown in FIG. 2. In order to show more clearly the relationship between the structures illustrated in FIGS. 1 and 2, like reference numerals have been used for corresponding parts. The bottom-gate structure illustrated in FIG. 2 comprises a gate electrode 12 deposited on a substrate 1 with an insulating layer 10 of dielectric material deposited thereover. Source and drain electrodes 2, 4 are deposited over the insulating layer 10 of dielectric material. The source and drain electrodes 2, 4 are spaced apart with a channel region 6 located therebetween over the gate electrode. An organic semiconductor 8 is deposited in the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4.
The conductivity of the channel can be altered by the application of a voltage at the gate. In this way the transistor can be switched on and off using an applied gate voltage. The drain current that is achievable for a given voltage is dependent on the mobility of the charge carriers in the organic semiconductor in the active region of the device (channel between the source and drain electrodes). Thus, in order to achieve high drain currents with low operational voltages, organic thin film transistors must have an organic semiconductor which has highly mobile charge carriers in the channel.
Charge carrier mobility is a measure of how easily a carrier moves in a particular material. This property of an organic semiconductor is, however, often compromised by the contact resistance of an organic thin film transistor device. A higher contact resistance results in a higher proportion of the applied voltage dropping across the interfaces between the source and drain electrodes and the organic semiconductor material in the transistor channel region and, as a result, a lower bias across the channel region is achieved. A high contact resistance thus has the effect of a much lower current level being extracted from the device due to the lower bias across the channel region, which comprises the charge carrier mobility of the organic semiconductor material.
Conventionally, contact resistance in organic transistors is reduced by applying surface treatment layers to the source and drain electrodes prior to depositing the semiconductor film or changing metal to a higher work function as necessary to inject charges to the HOMO level (for a p-type material). Such treatment layers (typically self assembled monolayers applied from solution or vapour phase) are used to produce a dipole layer at the metal surface to effectively shift the work function of the source and drain contacts to align with the HOMO level in the semiconductor and therefore reduce the barrier for charge injection from metal to the semiconductor.
The range of electronic properties found amongst the transition metal oxides mean that they are particularly advantageous materials for controlling the work function and hence the charge-injection properties of an organic thin-film transistor (OTFT). WO 2007/005618 discloses the insertion of thin layers of transition-metal oxide such as MoO3 between the organic semiconductor layer and the source and/or drain contacts in OTFTs. This prior art document, however, uses thermal evaporation to deposit the transition metal oxide. This has considerable disadvantages in terms of its potential for increasing up to a commercial scale process as it is both costly and inefficient.
Although transition metal oxides exhibit advantageous properties when deposited on the surfaces of source and drain electrodes of OTFTs, they do not adhere very well thereto.
Since source and drain electrodes must have the ability to accept and inject holes or electrons, they are usually fabricated from metallic conductors such as copper, silver or gold. Gold in particular is used for high-quality surface-to-surface contacts. These materials are, however, fairly unreactive. Pure gold is a chemically unreactive metal, whilst silver and copper surfaces react easily in air to form an unreactive metal oxide surface. Hence the source and drain electrode surfaces do not chemically react with the transition metal oxide layer deposited thereon. Significantly, these issues with adhesion can result in a non-uniform or even a discontinuous layer of transition metal oxide being deposited, making it extremely difficult to obtain repeatable results with the OTFTs. Thus whilst a transition metal oxide can improve contacts between the electrodes and active layers of an OTFT, its poor adhesion to the source and drain electrode surfaces when deposited thermally according to the prior art can limit the performance thereof.
There is thus a need to find an improved way of depositing transition metal oxide layers on inert metal layers. We have surprisingly found that it is possible to overcome the problems associated with metal oxide layers deposited by a thermal evaporation process according to the art by using an ammonium thio-transition metal complex as an adhesion promoter to immobilise transition metal oxides onto inert metal surfaces. The motivation for using such a complex is primarily to overcome the poor adhesion between the source and drain electrode surfaces and the transition metal oxide layer. This complex enables the formation of a continuous layer of transition metal oxide where the underlying substrate is made of an inert metal such as gold, onto which the transition metal oxide layer does not adhere well.
There are no examples of these complexes being used as adhesion promoters for organic thin-film transistors in the prior art. Rather the focus of the prior art is on improving the intrinsic electrical properties of the organic semiconductor layer within the OTFT and towards the development of device fabrication techniques (“High mobility solution processed 6,13-bis(triisopropyl-silylethynyl) pentacene organic thin film transistors”; S. K. Park et al., APPLIED PHYSICS LETTERS 91, 063514 (2007); Henning Sirringhaus, Takeo Kawase, Richard H. Friend, Tatsuya Shimoda, M. Inbasekaran, W. Wu, and E. P. Woo; “High-resolution inkjet printing of all-polymer transistor circuits”, Science, 290, 2123-2126 (2000)). There has been no research published into the use of an adhesion promoter to immobilise a transition metal oxide layer onto the source and drain electrodes of an OTFT device.
Moreover adhesion of substrate layers in organic thin-film transistors is a crucial property to optimise because of the effect of poor adhesion on the performance of the device. There is therefore a need to find additional ways of improving the adhesion of transition metal oxide layers to inert metal surfaces in organic thin-film transistors. The present invention addresses this need.